The invention relates to a process for the production of clad hot rolled strip, wherein a steel slab as base material is cladded with a corrosion-resistant layer material and the composite body is hot rolled.
Such a process is known f.i. from German patent 1 752 496 according to which, however, the base and layer materials are connected and the hot rolled strip is produced in one manufacturing step, namely in the rolling operation. The rolling operation is performed in a planetary mill, in which the thickness of the cladding assembly consisting of the base and layer materials is isothermally reduced in one pass at over 80% reduction rate to final strip thickness.
The cladding assembly comprises two outer plates, for example, of unalloyed steel or mild steel, which form the base material. Disposed between these plates are two relatively thin plates, for example, of chromium-containing steel as the layer material, these plates being separated from one another by a thin, uniform layer of a separating agent. The cladding assembly thus formed is heated in a non-oxidizing atmosphere in a slab furnace to a hot rolling temperature of, for example, 950.degree. to 1050.degree. C. and then introduced by transport rollers into the planetary mill. To facilitate the handling of the cladding assembly and prevent the plates from becoming separated, the outer plates can be welded along its edges. The strip emerging from the planetary mill can be separated by cutting off the edges into two individual composite metal strips each containing the base material and the layer material of stainless steel.
The external layers of the base material of the composite metal strip can be provided with a coating metal, such as tin, zinc, cadmium, aluminum or other alloys. Furthermore, the composite metal strip can be subjected to a cold rolling treatment by another process.
This prior art process for the production of clad hot rolled strip has the following disadvantages:
it requires special rolling mills, specially adapted to cladding rolling; PA1 it presumes the rolling out of double assemblies and has limitations as regards to total thickness of reduction, due to the separating layer; PA1 as a result of the comparatively low rolling speeds of the planetary mill (maximum 2.4 m/s) the strip temperature decreases over the strip length; PA1 the properties profile of the strip material must be adjusted in a separate thermal treatment process; PA1 due to the thermal treatment process to adjust the mechanical and chemical corrosion properties, carburization reactions take place in the bonding zone between the base and the layer materials. PA1 0.02-0.25% carbon PA1 up to 2.5% manganese PA1 up to 0.06% aluminum PA1 up to 0.8% silicon PA1 rest iron and unavoidable impurities. PA1 up to 0.1% niobium (columbium) PA1 up to 0.25% titanium PA1 up to 0.7% molybdenum PA1 up to 1.0% chromium PA1 up to 1.0% nickel PA1 up to 0.5% copper PA1 up to 0.15% vanadium PA1 max. 0.1% carbon PA1 max. 4% manganese PA1 max. 4% silicon PA1 max. 35% nickel PA1 10-30% chromium PA1 max. 7% molybdenum PA1 rest iron and unavoidable impurities. PA1 max. 1.5% titanium PA1 max. 1.5% niobium (columbium) PA1 max. 5% copper PA1 max. 0.5% aluminum PA1 max. 0,5% nitrogen. PA1 max. 0.1% carbon PA1 max. 4% manganese PA1 max. 4% silicon PA1 max. 45% iron PA1 10-30% chromium PA1 max. 10% molybdenum PA1 rest nickel and unavoidable impurities. PA1 max. 1.5% titanium PA1 max. 1.5% niobium (columbium) PA1 max. 5% copper PA1 max. 0.5% aluminum PA1 max. 0.5% nitrogen PA1 existing conventional hot strip mill can be used; PA1 hot rolling of an explosion clad semi-finished product eliminates the limitations in total reduction ratio mentioned above in connection with double clad assemblies, the result being manufacturing possibilities for a clad strip manufacturing programme which is wide as regards combination of materials, strip thickness and coil weight; PA1 the strip temperature can be kept uniform in hot strip mills, due to the high rolling speeds controllable with the strip length; PA1 the properties profile of the strip material can be adjusted by a special combination of heating temperature, forming conditions and cooling immediately after the material leaves the last rolling stand, so that there is no need for a further thermal treatment. PA1 the clad hot rolled strip is suitable for further processing on continuous manufactoring lines, for example, in the production of tubes or for cold rolling; PA1 hot strip rolling allows the economical manufacture of large-size thin claddings with a close tolerance, resulting also in economic advantages for further processing; PA1 carburization reactions of the cladding are so limited as to be negligible in comparison with cladding by conventional rolling and thermal treatment.
In the production of clad plate it is known to connect the base material of fine-grained structural steels and the layer material of corrosion-resistant materials to one another by an explosion cladding operation. The resulting composite body is rolled out in a plate rolling mill (German Journals "Bander, Bleche, Rohre" 11, 1982, pages 324 to 328 and "Stahl and Eisen", 102, 1982, No. 12, pages 619 to 624, German Offenlegungsschrift 1 237 787, Japanese Publication "Nippon Kokan Technical Report Overseas", No. 49, 1987, p. 10-18).
However, in the production of clad plate the hot rolling operation must be followed by thermal treatment of the clad plate, to adjust the mechanical properties of the base material and improve the corrosion resistance of the layer material. During this thermal treatment carbon diffuses out of the base material into the layer material. In the carburized zones of the layer material any carbide formation has an adverse effect on the corrosion resistance of the layer material.
However, cladded products in the form of strip are increasingly required, for example, for the production of clad tubes, more particularly sourgas tubes, or as a semi-finished product for cold rolled strip production.
The use of heavy clad plates for these applications is disadvantageous, since individual plates cannot be used for the production of pipes on continuous rolling mills, e.g., for pipe welding installations with high frequency induction or for spiral pipe welding installations; moreover, single plates are unsuitable semi-finished products for cold rolled strip production.
It is therefore an object of the invention to provide a process by which the clad hot rolled strip has outstanding properties of strength and toughness in the rolled condition and by which there is no need for any subsequent thermal treatment of the hot rolled strip to improve the corrosion resistance of the cladding.